1. Field of the Invention
The present invention generally relates to the technical field of mobile communications, and particularly relates to transmitting apparatuses and methods which transmit a signal using a multi-carrier scheme.
2. Description of the Related Art
In a multi-carrier modulation scheme, a symbol period can be increased and a guard interval can be inserted, etc., to effectively reduce multi-path interference, which is especially problematic in wideband radio communications. An orthogonal frequency division multiplexing (OFDM) scheme is especially promising in future radio communications systems. However, in the multi-carrier modulation scheme such as an OFDM scheme, each of multiple sub-carriers is mapped to a modulated signal independently. Therefore, a transmit signal in the time domain may exhibit a high peak power. Therefore, a large transmission power amplifier back off must be maintained. This ends up becoming a particularly large problem at a mobile terminal, in which decreased power consumption becomes important. For decreasing a peak power, or more specifically a peak-to-average power ratio (PAPR), a number of methods have been proposed in the past.
For example, a PTS (partial transmit sequence) method, an SLM (selected mapping) method, and an interleaving, etc., provides for changing a transmission scheme (data modulation scheme, a bit pattern in the frequency direction, a symbol interleave pattern, etc.), making the peak power small. In these methods, how the transmission scheme is changed must be reported from the transmitter to the receiver. Therefore, there is a problem that a radio resource is, to a certain extent, consumed for the reporting (signaling), which causes a corresponding decrease in data throughput. This problem causes a reduction in advantages for the OFDM of realizing high frequency utilization efficiency and high-quality transmission even in a multi-path environment.
A method which does not require signaling of auxiliary control information (side information) such as the transmission scheme includes a clipping and filtering scheme. This scheme suppresses amplitude of a signal component having a peak power in the time domain, and filters, using a time domain filter, an out-of-band distortion caused thereby. Suppressing the amplitude corresponds to distorting an original signal. In other words, it may also be said that the clipping and filtering scheme superimposes an intentionally interfering signal to an original signal such that the peak power decreases. This method, which directly decreases the peak power, has a strong tendency to cause a signal waveform to deviate from the original waveform, which is not preferable from a viewpoint of a received error rate of a signal.
Moreover, a tone reservation scheme divides sub-carriers into those for data transmission and those dedicated to peak suppression, amplitude and/or phase being set such that only the peak voltage is suppressed in the latter. This method, which makes it possible to maintain the sub-carrier signal for the data transmission, is preferable from the point of view of a signal error rate. However, some sub-carriers which are not to be used in data transmission have to be secured, so that a corresponding amount of decrease in the data throughput occurs.
Schemes devised in light of these problems as described above include an ACE (Active Constellation Extension) scheme. The ACE scheme, which also assumes the clipping and filtering scheme, superimposes an intentional interference signal to the original signal. The ACE scheme is devised such that a symbol receive error rate does not degrade.
FIG. 1 shows a symbol constellation of a transmit signal used in a QPSK scheme. As shown, one each of the respective reference signal points S1, S2, S3, and S4 is shown for each quadrant. Distorting a signal waveform such that a peak power decreases corresponds to remapping a signal point being mapped in alignment with each reference signal point to a point which is somewhat distant from the reference signal point. In this case, what allows any remapping is the above-described clipping and filtering scheme. In the ACE scheme, a remapping to an area with a wavy line arrow and an area which is shaded is allowed, but remapping to others is prohibited. For example, remapping, to T1, a signal point which was mapped to the reference signal point S1 is allowed, but mapping the same to T2 is prohibited. For convenience of illustration, the shaded area is drawn such that it represents a bounded square, but theoretically corresponds to an unbounded area. Remapping in such an area makes it possible to distort a signal waveform (i.e., suppress the peak power) without impacting signal point symbol determination. This is because the results of a hard decision on the signal point T1 always reach the reference signal point S1. The ACE scheme as described above is disclosed in Non-patent document 1.    Non-patent document 1: B. S. Krongold and D. L. Jones, “PAR Reduction in OFDM via Active Constellation Extension,” IEEE Trans. on Broadcasting, Vol. 49, No. 3, pp. 258-268, September 2003.